feat: add POLLING_LIMIT_PER_HOST to limit concurrent requests per host

Each batch of feeds sent to the worker pool is now guaranteed to contain unique feed URLs.

When `POLLING_LIMIT_PER_HOST` is set, an additional limit is applied to the number of concurrent requests per hostname, helping to prevent overloading a single server.

Note: Additional requests may still be made during feed refresh. For example, to fetch feed icons or when the web scraper is enabled for a particular feed.

internal/worker/worker.go

@@ -32,6 +32,7 @@ func (w *worker) Run(c <-chan model.Job) {
 			slog.Int("worker_id", w.id),
 			slog.Int64("user_id", job.UserID),
 			slog.Int64("feed_id", job.FeedID),
+			slog.String("feed_url", job.FeedURL),
 		)
 
 		startTime := time.Now()